In avoiding the difficulties involved in conventional gas bag designs due to the use of polyamide fabrics coated with methyl rubber or silicones, non-coated textile fabrics according to a proposal in EP-A1-0454213 are employed for the production of gas bags. The necessary low gas permeability of the fabric can be achieved by densification, more particularly by shrinking, thermofixing and calendering the fabric. By employing low-shrinkage threads in the lower part of the gas bag and high-shrinkage threads in the upper part of the gas bag it can be achieved that after inflation of the gas bag the gas permeability of the lower part is greater than that of the upper part of the gas bag so that a defined escape of gas from the gas bag occurs on impact of the vehicle occupant.
EP-A1-0509399 describes a gas bag in which a gas-tight and a gas-discharging fabric part are provided, the gas-discharging fabric part being made of a fabric having a crepe or a modified barley-corn weave. This fabric consists preferably of a non-sized polyester yarn.
Furthermore, described in EP-A1-0599158 is a gas bag for vehicular restraint systems which consists of several fabric parts differing in gas permeability. All fabric parts of the gas bag are produced of a textile material of the same nature, more particularly having the same thread count, the gas permeability of the fabric being set in view of the stress ratio between the weft and warp threads when the gas bag is inflated and in view of the dynamic response of the gas permeability via the weave and the interweaving.
In the case of gas bags produced of a non-coated fabric as known hitherto there is, however, the risk of the hot gases released after the ignition of the propellant of a pyrotechnical inflator for inflating the gas bag as a result of a collision, resulting in damage to the gas bag fabric and thus changing the gas permeability thereof undesirably. In addition to this, the vehicle occupants are exposed to the risk of burn injuries due to the uncontrolled release of hot gases into the vehicle interior.